1. Field of the Invention
The invention relates to a nuclear magnetic flowmeter; specifically, on the one hand, to a nuclear magnetic flowmeter with a calibration device, and on the other, to a calibration method for a nuclear magnetic flowmeter.
2. Description of Related Art
A nuclear magnetic flowmeter includes, first of all, as necessary for operation, a measuring tube through which a multiphase medium can flow and which can be connected to an inlet tube which is located in the flow direction of the medium upstream of the measuring tube and to an outlet tube which is located in the flow direction downstream of the measuring tube.
The atomic nuclei of the elements which have a nuclear spin also have a magnetic moment whish is caused by the nuclear spin. The nuclear spin can be construed as an angular momentum which can be described by a vector, and accordingly, the magnetic moment can also be described by a vector which is parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus in the presence of a macroscopic magnetic field is aligned parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of the precession is called the Larmor frequency ωL and is proportional to the amount of the magnetic field strength B. The Larmor frequency is computed according to ωL=γ·B. In the latter γ is the gyromagnetic ratio which is maximum for hydrogen nuclei.
Measurement and analysis methods which use the property of the precession of atomic nuclei with a magnetic moment in the presence of a macroscopic magnetic field are called nuclear magnetic resonance measurement or analysis methods. Usually, the voltages induced by the precessing atomic nuclei under various boundary conditions in a sensor coil are used as the output variable for the measurement and analysis methods. One example for measuring instruments which use nuclear magnetic resonance are the nuclear magnetic flowmeters which measure the flow rate of the multiphase medium flowing through the measuring tube and analyze the medium.
The prerequisite for an analysis using nuclear magnetic resonance is that the phases of the medium which are to be analyzed can be excited to distinguishable nuclear magnetic resonances. The analysis can comprise the flow velocities of the individual phases of the multiphase medium and the relative proportions of the individual phases in the multiphase medium. Nuclear magnetic flowmeters can be used, for example, to analyze the multiphase medium extracted from oil sources, a medium which consists essentially of the phases crude oil, natural gas and salt water, all phases containing hydrogen nuclei.
The medium extracted from oil sources can also be analyzed with so-called test separators. They branch off a small part of the extracted medium, separate the individual phases of the medium from one another and determine the proportions of the individual phases in the medium. But, test separators are not able to reliably measure proportions of crude oil smaller than 5%. Since the proportion of crude oil of each source continuously drops and the proportion of crude oil of a host of sources is already less than 5%, it is not currently possible to economically exploit these sources using test separators. In order to also be able to exploit sources with a very small proportion of crude oil, correspondingly accurate flowmeters are necessary.
Nuclear magnetic flowmeters can meet the demands of a host of applications, such as, for example, in the measurement of the flow rate of the multiphase medium extracted from a source through the measuring tube and in the determination of the proportions of crude oil, natural gas and salt water in the medium. Proportions of crude oil less than 5% can also be measured with nuclear magnetic flowmeters.
So that nuclear magnetic flowmeters achieve the measurement accuracy which is necessary for many applications, calibration of the flowmeters in which the calibration parameters are determined is necessary. As in any other measuring instrument, in nuclear magnetic flowmeters repetition of the calibration is also necessary at regular time intervals in order to also continuously achieve the possible measurement accuracy. Calibration methods and calibration devices known from the prior art call for the flowmeter which is to be calibrated to be connected to a calibration device and to be calibrated by means of a calibration method.
The calibration device is made to produce various known flow rates of various media through the measuring tube of the flowmeter. Media can be single-phase or multiphase media with known proportions of the individual phases in the respective medium, especially the knowledge of the nuclear magnetic resonance properties of the media being necessary. A calibration method can comprise measurements by the flowmeter at different flow rates with different media. The calibration parameters of the flowmeter can be obtained from the measured values of the measurements and the pertinent known flow rates and properties of the media.
The disadvantage of the calibration methods and calibration devices known from the prior art is that the nuclear magnetic flowmeter which is to be calibrated must be dismounted from the measurement site at regular time intervals and transported to the calibration device, a process which is, on the one hand, complex and expensive and which, on the other, disrupts operation at the measurement site.